Field of the Invention
The present invention is broadly concerned with VOC-free protective coatings that can be used to protect surfaces, such as those on bowling lanes.
Description of the Prior Art
Protective coatings are utilized in many products, including furniture, appliances and other electrical devices, floors, automotive exterior paint, and automotive interior parts. The coating usually protects plastic, metal, and wooden surfaces from being scratched as well as from other damage. The coating also conceals some of the underlying surface imperfections, makes the surface look smoother, and gives a glossier or more matte finish depending on the desired final product look.
Many types of coatings have been developed over the years, including two-part, ambient cure, and radiation curable types. A two-part is a coating that must be mixed with a hardener in order to be cured. An ambient cure coating is a coating where either solvent evaporation or moisture solidifies the coating. Radiation curable coatings include both ultraviolet (UV) light and electron beam (EB) curable coatings and cure by initiating reactions via irradiation. UV light is an electromagnetic radiation with a wavelength shorter than visible light wavelength. When the term UV light is applied to radiation curable conventional coatings, the wavelength of UV light is in the region of 100 nm through 400 nm.
Since their development, radiation curable coatings have gained acceptance due to their almost immediate cure or very short cure times, minimal oven use, and ability to be applied onto thermally unstable substrates. In order to cure common radiation curable coatings, UV mercury vapor lamps, which can be arc or microwave powered, are utilized. These lamps emit radiation in UVA, UVB, and UVC regions of the electromagnetic spectrum. This radiation is useful in the coating cure process. Besides UV radiation, these lamps produce a large quantities of infrared (IR) radiation (heat) as well as ozone.
A new field in radiation cured coatings that is emerging now is light emitting diode (LED) curable coatings. There are many advantages to using LED light sources over conventional UV mercury lamps. Some of the important advantages include that LEDs require much less power to run, they are instant on/off light sources, and there is no need for bulky cooling systems. In addition, LEDs have a much longer life compared to mercury vapor lamps. Moreover, current commercially available UV LEDs generate neither ozone nor excessive heat that may damage the coating being cured. Current commercially used UV LEDs emit radiation in the close to visible part of UVA (320-400 nm) and visible portion (400-700 nm) of the spectrum.
High energy UV radiation such as UVC (100-290 nm) is required to effectively cure standard UV coatings that react through free radical mechanisms such as various acrylate coatings in ambient conditions. Ambient conditions herein are defined as the presence of oxygen at concentrations that are equal to or above 20% by volume, which can be translated into 18 kPa oxygen partial pressure. In these conditions, most of the existing free-radical UV curable coating will not be sufficiently cured under UVA LED light even if UVA absorbing initiators are applied. The surface of the coating does not completely cure, i.e., it remains tacky after UVA exposure due to oxygen inhibition. The practice to apply a nitrogen blanket in order to properly cure these UV coating compositions is customary even when mercury vapor UV lamps are utilized. This is true especially if high scratch or abrasion resistance is required.
Coatings that cure through a cationic polymerization route such as those bearing epoxy functionality do not have the downside of the aforementioned property. Most of the commercially available cationic photoinitiators do not efficiently absorb light in UVA region. The addition of a UVA photosensitizer solves that problem. Therefore, the development of UV LED epoxy coatings looks like a promising path. Unfortunately, the shortcomings of this route include the yellow/brownish color of the resulting UV-cured epoxy coating, and reduced selection (if compared to available free radical) of low molecular weight highly crosslinkable monomers that are not viscous in order to formulate a 100% solids coating. The Environmental protection agency (EPA) VOC regulations are getting more strict each year, especially for indoor coatings and finishes. Therefore, the need exists for a 100% solids or water-based coating that can be cured in ambient conditions with UV LED light.